1H, 13C, 35Cl, and 81Br NMR of aqueous hexadecyltrimethylammonium salt solutions: Solubilization, viscoelasticity, and counterion specificity

Abstract

A combination of 1H, 13C, 35Cl, and 81Br NMR is used to investigate counterion specificity and solubilization behavior of hexaldecyltrimethylammonium micelles. The concentration dependence of the quadrupole relaxation of 81Br − and 35Cl − counterions in aqueous solutions of hexadecyltrimethylammonium bromide (CTAB) and chloride (CTAC), respectively, is found to be quite different and this can be referred to the formation of large elongated micelles in the former case but not in the latter. This striking difference is maintained in solutions containing both CTAB and CTAC. This is attributed to the coexistence of two types of micelles and a considerable specificity in the counterion binding, the Br − ions having a preference for the cylindrical and the Cl − ions for the globular micelles. For mixtures of CTAB with tetradecyltrimethylammonium chloride (TTAC), the same type of counterion specificity in 35Cl and 81Br NMR was observed as for mixtures of CTAB and CTAC. The 13C chemical shifts are closely the same for solutions of either CTAB, CTAC, or TTAC, but in mixtures of CTAB + TTAC or CTAC + TTAC there is a marked splitting of the ω-CH 3 resonance into two peaks. This is referred to alkyl chain packing disturbances in mixed micelles of two amphiphiles with different alkyl chain lengths. 1H and 13C NMR were employed to study the solubilization site for viscoelastic solutions of CTAB and 1-methylnaphthalene. A highly variable effect along the amphiphile chain was observed for both 1H and 13C relaxation and shielding. It could be established that solubilization of 1-methylnaphthalene occurs toward the polar part of CTAB micelles, about seven methylenes being affected appreciably. Chain flexibility is found to be influenced in a complex way by solubilization.